Electric cartridge type heaters have been known for many years. They usually have at least one metallic jacket, in the interior space of which at least one resistance wire is arranged, wherein an undesired electric contact between the resistance wire and the metallic jacket is prevented by the space between the metallic jacket and the resistance wire being filled at least partially with an electrically insulating material having good thermal conductivity, e.g., magnesium oxide. Electric cartridge type heaters also comprise, in particular, variants with an inner metallic jacket and an outer metallic jacket, often in the form of concentrically arranged tubes, between which the at least one resistance wire is arranged, so that an electric cartridge type heater designed in this manner can be pushed over an object to be heated.
It should be noted, furthermore, that an electric ballast resistor has a design identical to that of an electric cartridge type heater and an electric ballast resistor therefore also represents an electric cartridge type heater in the sense of this invention.
Different embodiments of such electric cartridge type heaters are known, in principle, especially those in which the electric contacting takes place from both sides, and those in which the electric contacting takes place from one side.
Electric cartridge type heaters have hitherto been manufactured mainly according to two different methods.
The Oakley principle is frequently used especially in electric cartridge type heaters in which the electric contacting takes place from both sides. The resistance wire, which is usually coiled tightly, pulled forward or sometimes also prestressed, is inserted into the jacket tube and clamped with its ends in pulling rods, so that the individual coils of the resistance wire are brought to spaced locations from one another. A filling tube is now pushed within the jacket tube over the resistance wire, which ensures that there can be no electric contact between the resistance wire and the jacket tube. When the usually powdered or granular electrically insulating material is filling in, the filling tube can then be pulled out of the jacket tube slowly while shaking, which causes the powdered or granular electrically insulating material to fill the volume released by the filling tube during pulling in just as the space between the coils, so that a sufficient insulation resistance and high-voltage resistance is guaranteed between the resistance wire and the metallic jacket or between two coils of the resistance wire.
The direct application of this principle is not possible in case of electric cartridge type heaters in which the electric contacting is to take place from one side, only because it presupposes that the resistance wire can be mechanically stressed during the filling of the electric cartridge type heater with the insulating material. This applies to high-performance cartridge type heaters, in which high power densities must be reached, so that very short distances from the outer wall and very small coil pitches are important.
To guarantee the desired course of the resistance wire, a carrier structure is therefore used, which provides the opposing forces to the acting mechanical stress, for example, in the form of a coil body usually manufactured from ceramic, or a carrier structure is used, which holds the end of the resistance wire located farthest away from the connection side, so that a mechanical stress, e.g., due to pulling rods, can be built up against this holding point, which stress is necessary to produce a coil structure coiled with spaces and to maintain the coils at spaced locations from one another, i.e., to avoid variations in spacing or short circuits between the individual coils.
However, it is seen in practice that the arrangement of the resistance wire on a coil body or such a carrier structure entails a great effort in terms of manufacturing technology and is susceptible to problems and is expensive.
These problems can be explained especially well for the case in which low-ohmic resistors must be obtained for the application for which the electric cartridge type heater is intended, which is frequently the case in case of relatively long electric cartridge type heaters.
In principle, a lower resistance is known to be able to be achieved by increasing the cross section of the hot wire. However, this leads not only to problems if a reduced space is required for construction, but also to stronger forces to be applied when feeding and removing the coil body into and from the coiling machine and when inserting and tightening the resistance wire in the hole of the coil body, which may even lead to breakage of the coil body in case of porous ceramic coil bodies and if automated coiling is employed, it reduces the speed with which the coiling machine can be operated and thus lowers the output or requires the simultaneous use of a larger number of expensive coil body coiling machines.
As an alternative, it is possible to change over to resistance wires with a lower resistivity, for example, by using CuNi44 instead of NiCr8020. However, this massively reduces the service life of the electric cartridge type heaters and the loadability of the ballast resistors.
The consequence of these problems in practice is usually that several individual resistors with higher resistance values coiled on coil bodies are manufactured and connected in parallel to achieve lower resistances in order to make it possible to avoid compromises in terms of both the cross section and the resistivity of the resistance wire. However, this also leads to a great effort, because a larger number of electric contact points must be provided and centering pieces must often be provided between the coil bodies to guarantee accurate positioning. In addition, any additional electric contact implies an additional risk of the electric cartridge type heater not functioning corresponding to the specifications, especially in case of low-voltage applications, because even low contact resistances may lead to interruptions.